fix: shorebird preview for iOS fails if no Flutter commands have previously run

[bryanoltman]

Because Flutter downloads artifacts on demand, and because shorebird preview does not execute any Flutter commands but relies on the existence of Flutter artifacts (specifically libimobiledevice for iOS), preview fails when those artifacts are not present.

This can be fixed by running a shorebird release command or any other command that causes Shorebird's Flutter to download artifacts, but this shouldn't be necessary.

The command and error output:

bryanoltman@moltman ~/Documents/sandbox/smoke (main) ✗
⑆ shorebird preview
✓ Fetching releases (0.2s)
Which release would you like to preview?
❯ ◉  1.0.0+21
  ◯  1.0.0+20
  ◯  1.0.0+18
  ◯  1.0.0+17
  ◯  1.0.0+16
  ◯  1.0.0+15
  ◯  1.0.0+13
  ◯  1.0.0+12
  ◯  1.0.0+11
  ◯  1.0.0+10
  ◯  1.0.0+9
  ◯  1.0.0+8
  ◯  1.0.0+7
Which release would you like to preview? 1.0.0+21
✓ Using production track (12ms)
✓ Using device Bryan Oltman’s iPhone (1.2s)
Unhandled exception:
ProcessException: No such file or directory
  Command: /Users/bryanoltman/shorebirdtech/_shorebird/shorebird/bin/cache/flutter/a6d1747d7f573b2ba2e2b96db1b76ed2f3f024da/bin/cache/artifacts/libimobiledevice/idevicesyslog -u 00008110-000265A10E92801E --network
#0      _ProcessImpl._start (dart:io-patch/process_patch.dart:402:33)
#1      Process.start (dart:io-patch/process_patch.dart:38:20)
#2      ProcessWrapper.start (package:shorebird_cli/src/shorebird_process.dart:289:20)
#3      ShorebirdProcess.start (package:shorebird_cli/src/shorebird_process.dart:132:27)
#4      IDeviceSysLog.startLogger (package:shorebird_cli/src/executables/idevicesyslog.dart:79:41)
#5      Devicectl.installAndLaunchApp (package:shorebird_cli/src/executables/devicectl/devicectl.dart:180:48)
#6      PreviewCommand.installAndLaunchIos (package:shorebird_cli/src/commands/preview_command.dart:379:43)
<asynchronous suspension>
#7      CommandRunner.runCommand (package:args/command_runner.dart:212:13)
<asynchronous suspension>
#8      ShorebirdCliCommandRunner.runCommand (package:shorebird_cli/src/command_runner.dart:208:18)
<asynchronous suspension>
#9      ShorebirdCliCommandRunner.run (package:shorebird_cli/src/command_runner.dart:124:14)
<asynchronous suspension>
#10     main (file:///Users/bryanoltman/shorebirdtech/_shorebird/shorebird/packages/shorebird_cli/bin/shorebird.dart:28:5)
<asynchronous suspension>